Science Literature

Coastal waters provide about 90% of global fish stocks, but significant geographical variations have been noted. For example, the subtropical eastern boundary coastal waters off the Americas are highly productive, but similar coastal waters off Namibia support comparatively meager fish populations.

"This apparent anomaly has been attributed to the episodic occurrence of hydrogen sulphide gas, which is toxic to economically important fish in the southwest African shelf waters. In fact, scientific reports of massive fish mortality in the Namibian coastal waters associated with sulphidic waters date back more than half a century."

A large patch of discoloured surface waters off Namibia attributed to toxic hydrogen sulphide release from the seafloor. (Credit: Image: Jacques Descloitres, MODIS Rapid Response Team, NASA/GS. Source here.)

Despite the potential economic importance, very little was known of these sulphidic events and of any mechanisms of detoxification. This has now changed. During 2004, the RV Alexander von Humboldt monitored a sulphidic event affecting about 7000 square kilometres and took water samples at various locations and depths. They found that microbial life was responsible for detoxifying the waters.

"[T]he detoxification [. . .] was mainly catalysed by two discrete populations of gamma- and epsilon-proteobacteria. Chemolithotrophic bacteria, accounting for ~20 per cent of the bacterioplankton in sulphidic waters, created a buffer zone between the toxic sulphidic subsurface waters and the oxic surface waters, where fish and other nekton live. This is the first time that large-scale detoxification of sulphidic waters by chemolithotrophs has been observed in an open-ocean system."

The key observation was that the sulphidic waters were detoxified without the presence of oxygen. There was no chemical oxidation. The researchers concluded that biological processes must have been involved, and when they looked for microbes, they found them. Known as chemolithotrophs (because they live on inorganic materials), these bacteria had previously been known only near black smokers. Their food comes from the hydrothermal vents, rich in hydrogen sulphide, that emerge from mid-ocean ridges. The researchers carried out lab tests to check that the bacteria could do the work of detoxification:

"Pure cultures of nitrate-reducing, sulphide-oxidizing bacteria have doubling times as short as ~1.5 h, indicating that these chemolithotrophs are capable of creating blooms under the correct conditions. The combined results indicate that the observed bloom of chemolithotrophs was sufficient to create a buffer zone between the oxic environment - where fish and other nekton live - and the toxic sulphidic Namibian shelf waters, by oxidizing sulphide with nitrate."

Looking more generally at ecological systems, it is apparent that there are many, many feedback loops - some positive and some negative - to consider. Simplicity exists only in the textbooks. In this case, there are disastrous emissions of a toxic gas that bring mass mortality to marine animals, but the mechanism is there to restore the system and allow recolonisation of the devastated areas. So effective is this mechanism that many events like this may be undetected by human observers:

"On the basis of our results from Namibia, we postulate that many sulphidic events in coastal waters may go unnoticed because bacteria consume sulphide before it reaches the surface."

The problem we face is that the degree of complexity is never fully appreciated. We are continually learning more about the biosphere and, by now, we should be recognising that we have still much more to learn. It is a human trait that we want to control our environment (rather than consider ourselves a part of it), and we like to think that we have us the tools to achieve our goals. A good example is iron-induced carbon sequestration. Research in this area has been quite extensive, and many are calling for more and more resources to pursue the goal of terraforming our own planet. Happily, new research, reported in the same issue of Nature as the detox research, has questioned the wisdom of taking this forward. One oceanographer is quoted as saying: "Ocean iron fertilization is simply no longer to be taken as a viable option for mitigation of the CO2 problem". (Restricted link here)

Why is this relevant to design in nature? It is because the ability of ecosystems to recover from major perturbations continually challenges the idea that these systems are the product of undirected evolutionary tinkering. The feedback systems are pervasive, and they interact in ways that defy simplistic modeling. Design is suggested here at the level of hypothesis. What if we infer design: will this help or hinder policy? The null hypothesis, that there is no design, is already dominant. However, it is leading to oversimplification and much funding is being spent trying to run before we can walk (e.g. iron-induced carbon sequestration). The design hypothesis suggests we try to work with the natural world, not try to control it.

Another tendency stimulated by the null hypothesis is to regard the Earth's ecosystems as fragile and easily devastated by change. A good example of this is climate modelling, where the world's most powerful computers are harnessed to predict the future. However, the outputs are only as good as the inputs, and our knowledge of climate is small compared with our ignorance. In order to replicate some of the dramatic climate changes that have occurred in times past, the researchers have found it necessary to construct delicately balanced models so that the climate goes into a runaway mode (either to freeze or to roast). A design perspective is sceptical of these finely-tuned models. They are over-sensitive to perturbations, not because of empirical evidence, but because the modellers need to explain hot and cold periods in Earth history. The empirical evidence, as illustrated by the detox research reported above, is that there are robust feedback mechanisms. If the 'robust biosphere' hypothesis is correct, then there must be better ways of spending climate research money.

Detoxification of sulphidic African shelf waters by blooming chemolithotrophs
Gaute Lavik, Torben Stuhrmann, Volker Bruchert, Anja Van der Plas, Volker Mohrholz, Phyllis Lam, Marc Mu mann, Bernhard M. Fuchs, Rudolf Amann, Ulrich Lass & Marcel M. M. Kuypers
Nature 457, 581-584 (29 January 2009) | doi:10.1038/nature07588

Coastal waters support ~90 per cent of global fisheries and are therefore an important food reserve for our planet. Eutrophication of these waters, due to human activity, leads to severe oxygen depletion and the episodic occurrence of hydrogen sulphide - toxic to multi-cellular life - with disastrous consequences for coastal ecosytems. Here we show that an area of ~7,000 km2 of African shelf, covered by sulphidic water, was detoxified by blooming bacteria that oxidized the biologically harmful sulphide to environmentally harmless colloidal sulphur and sulphate. [. . .] This is the first time that large-scale detoxification of sulphidic waters by chemolithotrophs has been observed in an open-ocean system. The data suggest that sulphide can be completely consumed by bacteria in the subsurface waters and, thus, can be overlooked by remote sensing or monitoring of shallow coastal waters. Consequently, sulphidic bottom waters on continental shelves may be more common than previously believed, and could therefore have an important but as yet neglected effect on benthic communities.

See also:

Living dangerously: Implications of hydrogen sulphide for marine life along the Namibian coast (pdf presentation)
B. Currie, K. R. Peard, V. Bruchert, K-C. Emeis,R. Endler, A. Salvanes, A. C. UtnePalm, S. Weeks, A. Bakun,R. Bahlo, A. Goosen.
Namibian Ministry of Fisheries and Marine Resources, Max Planck Institute for Marine Biology.